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I am trying to figure out how the reactivity series comes about. My understanding is that elements with a higher electronegativity will be more reactive than elements with a lower electronegativity, and that elements with a low ionization energy will be more reactive than elements with a high ionization energy.

Here is a chart of electronegativity (from Wikipedia):

This shows electronegativity decreasing (as reactivity increases) down Group 1 and it also decreasing (as reactivity decreases) down Group 7.

Here is a chart of ionization energy:

enter image description here
(source: genchem at www.dartmouth.edu)

This shows ionization energy decreasing (as reactivity increases) down Group 1 and it also decreasing (as reactivity decreases) down Group 7.

Neither electronegativity nor ionization energy can predict both reactivity trends, so what is going on?

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    $\begingroup$ Two thoughts: 1. More reactive with what? 2. The alkali metals are very power reducing agents - their reaction with water gets more explosive as you go down the group (to lower electronegativities). $\endgroup$
    – Ben Norris
    Commented Dec 2, 2014 at 11:35
  • $\begingroup$ @BenNorris I guess this question is referring to the metal reactivity series. The WP page says that: "It is used to summarize information about the reactions of metals with acids and water, double displacement reactions and the extraction of metals from their ores." $\endgroup$ Commented May 16, 2018 at 12:27

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With less electronegativity, there is also less ionization energy. This means for metals, it is easier to lose the electrons. When it is easier to lose the electrons, the metal becomes more reactive because it uses less energy to fulfill its goal; to gain a full outer shell.

For nonmetals, the main goal is to gain electrons and not lose them. Thus, an increase in electronegativity and ionization energy means an increase in reactivity because the nonmetals want to hold on to their electrons and gain more vigorously to achieve the full outer shell.

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Reactivity is not only about the tendency of an element becoming an ion( as reflected in ionisation energy and electronegativity). Reactivity takes into account the stability of the produced ion. Often reactions that take place in an aqueous solution takes into account the stability of the ion in water.

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Following is from Wikipedia

Electronegativity, symbolized as χ, is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond.1 An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus.

Whereas an element's reactivity is based more on how ready it is to gain or lose electrons. Here is a visual explanation of the situation.

enter image description here

As the atoms move from left to right, the electronegativity of these atoms are decreasing as the distance between the electrons and the positively charged nucleus is would be greater, giving it lower ability to attract electrons. However, as you said the reactivity series says potassium is more reactive than sodium. Let me explain...

The potassium would have a lower tendency of attracting electrons compared to sodium, it would hence more readily lose electrons to form potassium ions (K+) to form ionic bonds with other elements. The reaction would tend to occur more readily hence the reactivity of potassium is higher than sodium.

There are also other cases in the reactivity series that are explained in a similar way. Eg. Sodium and Magnesium. (For this you need to consider only the net charge of the ions formed as the charge would be considered neutral once electrons and protons are equal). The magnesium ion has charge of 2+ while Na has a charge of 1+, this would mean that the there is stronger attraction between the valence electrons of Mg than the valence electrons of Na. The magnesium will lose the electrons less readily and so it will be placed lower than sodium in the reactivity series.

The reason why it is different for the non metallic elements is that they need to gain electrons in order to react and form bonds. This would mean that a higher electronegativity would give it a higher ability to gain electrons and hence react more readily, placing them higher in the reactivity series, unlike the metallic elements that would go through a different scenario.

In summary, the electronegativity of an atom affects the reactivity whether it needs to gain or lose electrons to react. Hope this answers your question!

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